Agricultural Practices and Their Impact on Spatial Distribution of Soil Parameters.

Since earlier soil fertility surveys in 1908 and the late 1960’s, conservation tillage has been widely adopted in Illinois. Our objective was to better understand current soil fertility status across Illinois by inventorying agricultural soils in a spatially-explicit manner. A total of 547 soil samples were randomly collected prior to corn harvest across the state in 2007 and 2008. A six-core (2-cm diameter) composite sample divided into 0–8 and 8–18 cm depth increments was taken from each field within a 3-m diameter georeferenced area.  Samples were analyzed for Bray P1, ammonium acetate–extractable K, Ca, and Mg, pH, and OM. Semivariogram analysis was used to study the spatial variability of soil properties and ordinary kriging was used to interpolate the point-based observations. Magnesium and OM had strong spatial dependence in both layers (0-8, 8-18 cm), Ca had moderate spatial dependence, whereas P, K, and pH had moderate to weak spatial dependence. Compared with surface soils where the P and K variability is dominated by nugget (non-spatially structured variability), P in subsurface soils exhibited greater partial sill (spatially structured variability) and sill (total variability). This evidences the vertical stratification of soils resulting from conservation tillage practices. Calcium and Mg also showed strong spatially-structured variability which accounts for approximately 60 and 98% of their overall variability, respectively. The ordinary kriging maps showed that the spatial patterns of Ca, Mg, and OM concentration generally matched the boundaries of three glacial periods that corresponds to the degree of soil weathering in Illinois. Conversely, P, K, and pH did not exhibit such strong north-south trend. These results indicate that Ca and Mg were predominantly controlled by natural soil forming factors such as weathering of minerals rather than anthropogenic activities such as liming, while P, K, and pH were largely influenced by management.